Solid state rechargeable lithium battery, stacking battery, and charging method of the same

ABSTRACT

A solid-state secondary lithium battery with excellent charge and discharge cycle characteristics, using a negative electrode active material which shows discontinuous change of potential caused by the lithium ion insertion and extraction reactions, wherein the amount of the lithium ion inserted, until discontinuous change of potential of the negative elctrode takes place, is equal to or smaller than the maximum amount of extraction of lithium ions within the range where lithium ions are inserted and extracted into or from the lithium transition metal oxide reversibly, and a battery assembly using these batteries.

FIELD OF THE INVENTION

The present invention relates to a solid-state (secondary) lithiumbattery (rechargeable lithium battery) using a lithium ion conductivesolid electrolyte, a battery assembly (a stacking battery), and a methodfor charging these batteries.

BACKGROUND OF THE INVENTION

Development of portable electric and electronic devices such as,typically, personal computers and handy-phones in recent years hasprovoked a phenomenal increase of demand for batteries used as powersource of these devices. Especially lithium batteries have been studiedintensively in expectation of obtaining a battery capable of providing ahigh energy density as lithium has small atomic weight and highionization energy, and such lithium batteries have now come to be usedpopularly as a power source of the various portable electric andelectronic devices and for various other purposes.

On the other hand, with such prevalence of lithium batteries, increasingconcern has been shown recently on safety of the batteries in practicaluse thereof, in view of the enlargement of inherent energy due to theincreasing amount of the active material contained in the battery andthe increase of the content of the organic solvent which is aninflammable material used for the electrolyte. Use of a solidelectrolyte, which is a nonflammable material, in place of theconventional organic solvent electrolytes is very effective for securingsafety of the lithium batteries, and the development of a solid-statelithium battery with high safety feature has been required.

For obtaining a high-voltage battery, a specific material such aslithium cobalt oxide (Li_(1-x) CoO₂) is used as the active material forthe positive electrode of a lithium battery. This material is of ametastable phase that can be formed as a result of extraction of lithiumions from LiCoO₂, which is a high temperature stable phase. LiCoO₂ has astructure in which the respective triangular lattices of oxygen, lithiumand cobalt are accumulated in the order of O--Li--O--Co--O--Li--O, withlithium ions present between the CoO₂ layers. Said material can serve asan electrode material of a lithium battery as reversible insertion andextraction of lithium ions take place between said layers.

The lithium ions in LiCoO₂ play the role of having the CoO₂ layersattracted to each other by virtue of electrostatic attraction betweenthe cationic lithium atoms and anionic oxygen atoms. When the lithiumions are extracted from LiCoO₂, since there no longer exists Li in theO--Li--O structure, electrostatic repulsive force between the oxygenatoms in the CoO₂ layers elevates to cause an interlaminar stretch.Consequently, there takes place expansion or shrinkage of the crystallattices due to the lithium ion insertion/extraction reactions duringcharging or discharging of the lithium battery.

The interface between the electrode active material and the electrolytein a solid-state battery using a solid electrolyte is a solid/solidinterface which, as compared with the solid/liquid interface in theconventional liquid electrolyte batteries, has greater difficulties inenlarging the contact area between the electrode active material and theelectrolyte, namely the electrochemical reaction interface. Further, incase a material which undergoes a volumetric change during charging ordischarging, such as the afore-mentioned lithium cobalt oxides, is usedas the electrode active material, it is difficult to keep a steadyinterface between such electrode active material and the solidelectrolyte. Consequently, the interface is always subject to changeduring operation of the battery, and the change of the interface causesa corresponding change of overvoltage in the electrode reaction.

Charging of a battery can be effected by either a constant-currentcharging method or a constant-voltage charging method. In the case ofthe constant-current charging method, charging is terminated when theterminal voltage of the battery has reached a certain value, so that itis necessary to set the charging voltage of the battery no matter whichmethod is used.

However, as explained above, the overvoltage of the reaction changesincessantly at the electrode of a solid-state battery, so that theelectrode potential on during charging is not constant even in case theterminal voltage of the battery is kept constant during charging. Thismeans that the battery may be charged deeply in case the reactionovervoltage decreases even though charging is performed at a constantvoltage.

Li_(1-x) CoO₂ formed by extraction of lithium ions from LiCoO₂ shows ahigh equilibrium potential of 4 V Vs Li or above, but the crystalstructure becomes unstable due to the afore-mentioned repulsion forcebetween the oxygen atoms. Therefore, in order to effect stabilizedoperation of a battery using said material as the electrode activematerial, it is necessary to limit the maximum amount of extractionlithium ions within a reversible range. Excessive elimination of lithiumions causes a change of crystal structure, making it unable to show thereversible insertion/extraction reactions of lithium ions. Therefore, ina charge and discharge cycle of a solid-state lithium secondary batteryusing said material as the electrode active material, when there takesplace a decrease in overvoltage such as mentioned above and the batteryis charged deeply, extraction amount of lithium ions exceeds the limitof reversible range, resulting in deterioration of theinsertion/extraction reactions of lithium ions between the crystallayers. This leads to a decrease of battery capacity with the charge anddischarge cycle of the battery, giving rise to the problem of reducedcharge and discharge cycle life of the battery.

Said phenomenon of deep charging of the battery is also caused byinaccuracy of charging control of the charger or drift of chargingvoltage with time. Therefore, precise charging control is essential forelongating the cycle life of the battery. This is, however, hardlyachievable by use of an uncostly charger, giving rise to the problem ofnecessity of using a costly charger.

The foregoing explanation concerns the case where LiCoO₂ was used as thepositive electrode active material, but similar problems may arise evenin the case of a solid-state lithium secondary battery using othermaterials such as Li_(1-x) NiO₂ and Li_(1-x) Mn₂ O₄ as the positiveelectrode active material since these materials also show an equilibriumpotential exceeding 4 V when taking a metastable phase.

There are other factors, such as volumetric change of the electrodeactive material consequent to charging or discharging, which greatlyaffect the cycle characteristics of a solid-state battery. Generally, asolid-state battery is composed of solid particles, and these solidparticles become plastic on occurrence of displacements such asexpansion or contraction in case no force is acting in the direction ofcausing aggregation of the particles. Therefore, in a non-pressurizedsolid-state battery, there are formed the voids among the consistingsolid particles therearound due to expansion or contraction of theelectrode active material caused by charging or discharging. This iscausative of a decrease of the electrochemical reaction area afterrepetition of charge and discharge resulting in a reduction of currentcollectability of the electrode active material and degradation ofbattery performance.

The present invention is envisaged to solve the above problems andprovide a solid-state lithium secondary battery with excellent chargeand discharge cycle characteristics, a battery assembly, and a methodfor charging them.

SUMMARY OF THE INVENTION

The present invention relates to a solid-state (secondary) lithiumbattery comprising (a) an electrolyte layer mainly composed of a lithiumion conductive solid electrolyte, (b) a positive electrode containing alithium transition metal oxide capable of inducing reversibleelectrochemical insertion and extraction reactions of lithium ions intoor from lithium ion sites in the crystal structure, and (c) a negativeelectrode containing a material showing discontinuous change ofpotential consequent to the insertion and extraction reactions oflithium ions into or from the lithium ion sites in the crystalstructure, the amount of the lithium ion inserted, until potential ofthe material showing discontinuous change of potential consequent to theinsertion and extraction reactions of lithium ions discontinuouslychanges, being equal to or smaller than the maximum extracted amount ofthe lithium ions which is extracted by the reversible insertion andextraction reactions of the lithium transition metal oxide (the maximumamount of extraction of lithium ions within the range where lithium ionsare reversibly inserted and extracted into or from the lithiumtransition metal oxide).

The present invention, further, relates to a battery assembly comprisingplural electrically connected unit cells, which battery assembly has astructure of plural unit cells connected in series at a part of thebattery assembly and a unit cell having at least the smallest capacityamong the plural unit cells connected in series is the solid-statelithium secondary battery as mentioned above.

The present invention, further, relates to

a method for charging the battery as mentioned above comprising the stepof:

charging the battery showing discontinuous change of voltage from thevoltage V₁ to the voltage V₂ relative to a charged quantity ofelectricity, at a voltage same as or less than the voltage V₂.

The present invention, further, relates to

a (secondary) lithium battery in which an amorphous lithium ionconductive solid electrolyte mainly composed of a sulfide is used aselectrolyte, and Li_(4/3) Ti_(5/3) O₄ is used as electrode activematerial.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a diagram showing the change of the single-electrodepotential of the positive and negative electrodes for illustrating theoperational principle of the solid-state secondary lithium batteryaccording to the present invention.

FIG. 1(b) is a diagram showing the change of electromotive force of thebattery for illustrating the operational principle of the solid-statesecondary lithium battery according to the present invention.

FIG. 2(a) is a diagram showing the change of the single-electrodepotential of the positive and negative electrodes for illustrating theoperational principle of the solid-state secondary lithium batteryaccording to the comparative example.

FIG. 2(b) is a diagram showing the change of electromotive force of thebattery for illustrating the operational principle of said comparativesolid-state secondary lithium battery.

FIG. 3 is a cross sectional view of the solid-state secondary lithiumbattery in an embodiment of the present invention.

FIG. 4 is a diagram showing the charging curves of the solid-statesecondary lithium battery in an embodiment of the present invention andthat according to the comparative example.

FIG. 5 is a diagram showing the charge and discharge cyclecharacteristics of the solid-state secondary lithium battery in anembodiment of the present invention and that according to thecomparative example.

FIG. 6 is a diagram showing the charge and discharge cyclecharacteristics of the solid-state secondary lithium battery in anembodiment of the present invention and that according to thecomparative example.

FIG. 7 is a cross sectional view of the battery assembly in anembodiment of the present invention.

In the drawings, reference numerals designate the following:

1: positive electrode 2: solid electrolyte layer 3: negative electrode4: battery container 5: gasket 6: cover 7: unit cell (i) 8: unit cell(ii) 9: connector 10: battery container 11: gasket 12: cover

DETAILED DESCRIPTION OF THE INVENTION

The term "reversible" used in electrochemistry sometimes means that thereaction rate is high. But, hereinafter, the "reversibleinsertion/extraction reactions" means that lithium ions can be insertedelectrochemically to the lithium ion sites of the lithium transitionmetal oxides, from which once lithium ions were extractedelectrochemically.

In the present invention, as the material showing discontinuous changeof potential consequent to the insertion/extraction reaction of lithiumions into or from the lithium ion sites in the crystal structure, therecan be used at least one metal selected from the group consisting ofindium, aluminum, lead, bismuth, antimony, gallium, tin, silver,silicon, zinc, cadminum, arsenic and titanium, or an alloy of the metalsselected from the above group, or an alloy of lithium and at least onemetal selected from the above group.

In accordance with the present invention, a solid-state secondarylithium battery can be constructed by using indium as the materialshowing discontinuous change of potential consequent to theinsertion/extraction reaction of lithium ions into or from the lithiumion sites in the crystal structure, and by specifying the lithium ioninsertion and extraction such that the amount of the lithium ioninserted, until potential of the material showing discontinuous changeof potential consequent to the insertion and extraction reactions oflithium ions discontinuously changes, is the amount corresponding to thefollowing reaction formula (3):

    In+Li.sup.+ +e.sup.- →In-Li                         (3)

A solid-state secondary lithium battery can be also composed by usingaluminum as the material showing discontinuous change of potentialconsequent to the insertion/extraction reactions of the lithium ionsinto or from the lithium ion sites in the crystal structure, and byspecifying the lithium ion insertion and extraction such that the amountof the lithium ion inserted, until potential of the material showingdiscontinuous change of potential consequent to the insertion andextraction reactions of lithium ions discontinuously changes, is theamount corresponding to the following reaction formula (4):

    Al+Li.sup.+ +e.sup.- Al-Li

Further, in the present invention, as the material showing discontinuouschange of potential consequent to the insertion/extraction reactions ofthe lithium ions, there can be used a compound selected from the groupconsisting of transition metal oxides, lithium transition metal oxides,transition metal sulfides and lithium transition metal sulfides.

As the transition metal element of said transition metal oxides orlithium transition metal oxides, at least one element selected from thegroup consisting of titanium, manganese, tungsten and vanadium can beused.

As the transition metal element of said transition metal sulfides orlithium transition metal sulfides, at least one element selected fromthe group consisting of titanium, molybdenum, niobium, tungsten andvanadium can be used.

As the transition metal element of the lithium transition metal oxidesused for the positive electrode, at least one element selected from thegroup consisting of cobalt, nickel, manganese and iron can be used.

As the lithium ion conductive solid electrolyte, an electrolytecomprising an inorganic compound can be used.

As said inorganic compound, a sulfide-based amorphous inorganic compoundcan be used. The compounds synthesized from the materials mainlycomposed of lithium sulfide or silicon sulfide can be also usable.

In the present invention, in another embodiment thereof, theabove-described solid-state secondary lithium battery can be used as theunit cell having at least the smallest capacity in a plurality ofseries-connected unit cells in a battery assembly having a structure inwhich, at a part thereof, the plural unit cells are connected in series.

A plurality of unit cells can be housed in an airtight battery containerto constitute a battery assembly.

In the present invention, these batteries showing discontinuous changeof voltage from the voltage V₁ to the voltage V₂ with relation to thecharged quantity of electricity can be charged at a voltage below V₂.

Also, in the present invention, an amorphous lithium ion conductivesolid electrolyte mainly composed of a sulfide can be used aselectrolyte, and Li_(4/3) Ti_(5/3) O₄ can be used as electrode activematerial.

An embodiment of the present invention is described below by taking thecase of a solid-state secondary lithium battery using a lithium cobaltoxide as a positive electrode active material and an indium-lithiumalloy as a negative electrode active material.

In a solid-state secondary lithium battery constituted by using alithium ion conductive solid electrolyte, a lithium cobalt oxide aspositive electrode active material and indium as negative electrodeactive material, there take place a reaction of the following formula(5) at the positive electrode and a reaction of the following formula(6) at the negative electrode during charging. That is, lithium ions areextracted from the lithium cobalt oxide at the positive electrode whilean indium-lithium alloy is formed by the lithium ions extracted from thepositive electrode at the negative electrode.

    LiCoO.sub.2 →Li.sub.1-x CoO.sub.2 +xLi.sup.+ +xe.sup.-(5)

    In+xLi.sup.+ +xe.sup.- →In-Li.sub.x                 (6)

Here, the reversible insertion/extraction reaction of lithium ions intoor from the lithium cobalt oxide can be effected in the range wherex≦0.5, and the potential of the indium-lithium alloy changesdiscontinuously from about 0.6 V to about 0.4 V when x is about 1.0.

FIG. 1(a) is a diagram showing the change of single-electrode potentialat the positve and negative electrodes during charging of a solid-statesecondary lithium battery constructed by using as positive electrodeactive material a lithium cobalt oxide in an amount of 2.1 times molarquantity of indium used as negative electrode active material. Thepattern of change of electromotive force of the battery is shown in FIG.1(b). The maximum amount of extraction of lithium ions that can causereversible insertion and extraction of lithium ions into or from thelithium cobalt oxide is an amount that provides the composition of Li₀.5CoO₂, while the amount of lithium ions loaded until discontinous changeof indium potential takes place is an amount that provides thecomposition of In-Li. Therefore, this solid-state secondary lithiumbattery is so constructed that amount of the lithiun ion inserted untildiscontinuous change of indium potential occurs will be substantiallyequal to or slightly smaller than the maximum amount of extraction oflithium ions that can cause reversible insertion and extraction oflithium ions into or from the lithium cobalt oxide.

FIG. 2(a) shows the pattern of change of electrode potential at thepositive and negative electrodes during charging of a solid-statesecondary lithium battery constructed by using as positive electrodeactive material a lithium cobalt oxide in an amount of 1.4 times molarquantity of indium used as negative electrode material. The pattern ofchange of electromotive force of this battery is shown in FIG. 2(b). Inthis solid-state secondary lithium battery, amount of the lithium ioninserted until discontinuous change of indium potential occurs isgreater than the maximum amount of extraction of lithium ions thatcauses reversible insertion and extraction of lithium ions into or fromthe lithium cobalt oxide.

In these solid-state secondary lithium batteries, the maximum batterycapacity is provided until the lithium cobalt oxide takes thecomposition of Li₀.5 CoO₂, so that it should be possible to charge thebattery completely up to the point A in FIGS. 1 and 2 by carrying outcharging at a voltage (Vcell) corresponding to the difference(Epos--Eneg) between the positive electrode potential (Epos) and negtiveelectrode potential (Eneg) during the charging of the battery. Actually,however, in the battery being charged, there takes place DC polarizationdue to internal resistance or overvoltage (η) due to electrode reaction,and therefore it is not charged at the maximum battery capacity whencharged at the voltage of Vcell, so that it is necessary to chargebattery at a voltage (Vend) higher than Vcell for full charging of thebattery.

However, the value of η is not constant and tends to decrease inaccordance with charging of the battery as explained below.

For instance, when charging is performed at a constant voltage, ηdecreases with decreasing of the charging current near the end ofcharging.

Also, in case of using a lithium ion conductive solid electrolyte, theretakes place a reduction of η with charging of the battery according tothe following mechanism. At the positive electrode during charging, thedistance between the CoO₂ layers of lithium cobalt oxide elongates andthe volume of the positive electrode expands. Volume expansion alsotakes place at the negative electrode as metallic indium is converted toan indium-lithium alloy. Consequently, pressure to the battery elementincreases, which contributes to the improvement of contact between thesolid particles constituting the battery and the reduction of polosityof the battery element. This leads to a reduction of internal resistanceof the battery and a corresponding decrease of η.

Here, let us consider the case where η was made extremely small, thatis, the case of η→0.

When η→0 near the end of charging where the charging voltage is Vend,the difference between the positive electrode potential and the negativeelectrode potential, viz. Epos--Eneg, approaches Vend. Consequently, thebattery is charged deeply to the point B in FIG. 2, exceeding the pointA to which charging is supposed to be effected. In this case, lithiumions are extracted from the lithium cobalt oxide until it takes thecomposition of Li₀.4 CoO₂. This impairs reversibility of the lithium ioninsertion/extraction reactions in the lithium cobalt oxide, resulting indeterioration of the charge/discharge cycle characteristics.

On the other hand, in the case of the construction of FIG. 1 accordingto the present invention, if the value of Vend--Vcell is smaller thanthe amount of discontinuous change (ΔV) of indium potential, the batteryis only charged to the point B in the drawing even when η→0, and theamount of extraction of lithium ions from the lithium cobalt oxideremains in the range where reversible insertion and extraction can takeplace, even if charging is carried out at a voltage of Vend.

In case the above battery construction is adopted, even if the chargerprecision is low and the charging voltage elevates, if such variation isless than ΔV, the amount of the lithium ions extracted from the lithiumcobalt oxide is also kept within the range where reversible reactionstake place, and there occurs no deterioration of the batteryperformance.

According to the above-described mechanism and its operations, there canbe obtained an excellent solid-state secondary lithium batteryconsisting of an electrolyte layer mainly composed of a lithium ionconductive solid electrolyte, a positive electrode containing a lithiumtransition metal oxide which shows the reversible electrochemicalinsertion and extraction reactions of lithium ions into or from thelithium ion site in the crystal structure, and a negative electrodecontaining a material which shows discontinuous change of potentialconsequent to said lithium ion insertion and extraction reactions, saidsolid-state secondary lithium battery being prominently improved incharge and discharge cycle characteristics owing to the structuralfeature that the amount of the lithium ion inserted until potential ofthe material showing discontinuous change of potential consequent tosaid lithium ion insertion and extraction reaction changesdiscontinuously is equal to or smaller than the maximum amount ofextraction of lithium ions that causes reversible insertion andextraction reactions of lithium ions into or from said lithiumtransition metal oxide.

The battery of the above-described mechanism shows discontinuous changeof voltage from V₁ to V₂ in relation to the charged quantity ofelectricity as shown in FIG. 1(b). In this battery, therefore, it isdesirable that the charging voltage is set below V₂.

The above-described phenomenon of impairing the charge and dischargecycle characteristics by over-charging is also observed in the secondarylithium batteries using an organic electrolyte, however particularly ina solid-state secondary lithium battery using a solid electrolyte forthe following reason.

The mechanism of impairment of charge and discharge cyclecharacteristics by deep charging can be accounted for by the fact that,as already stated, when the lithium ions are extracted from LiCoO₂,since Li no longer exists in the O--Li--O structure, the electro-staticrepulsive force between the oxygen atoms in the CoO₂ layer strengthensto unstabilize the crystal structure. In the electrolyte using anorganic solvent, however, since the organic solvent molecules are alsoco-inserted between the CoO₂ layers, such organic solvent molecules arepresent between the layers even after the lithium ions have beenextracted from between the CoO₂ layers. The anionic oxygen atoms in theCoO₂ layers induce a quadrupolar moment in the organic solventmolecules, and the oxygen atoms and the organic molecules are attractedeach other. Consequently, the static repulsive force between the oxygenatoms decreases to diminish instability of the crystal structure.Because of the absence of such interaction in a solid-state secondarylithium battery, crystal instability is greater than when an organicelectrolyte is used, so that degradation of cycling performance due tothe deep charging is greater characteristics by deep charging.

For the reasons stated above, the effect of the present invention isoutstanding in a solid-state secondary lithium battery using a lithiumion conductive solid electrolyte.

The lithium ion conductive solid electrolytes may be divided into twogroups: inorganic compounds and organic polymer compounds. Those mainlycomposed of an organic polymer compounds contain an organic solvent asplasticizer, and the molecules of this organic solvent contribute to thereduction of instability of the crystal structure. Therefore, the effectof the present invention is particularly remarkable in a solid-statesecondary lithium battery using a lithium ion conductive solidelectrolyte composed of inorganic compounds.

Further, by using a lithium ion conductive solid electrolyte ofinorganic compounds and also having high ion conductivity, it ispossible to improve high rate capability of the battery. Therefore, asthe lithium ion conductive solid electrolyte an inorganic compoundshowing high ion conductivity, it is especially preferred to use theamorphous electrolytes mainly composed of sulfides. These electrolytescan be obtained by melting at a high temperature and then rapidlycooling a mixture of a sulfides for glass network forming and lithiumsulfide for producing mobile lithium ions in the glass structure. Ofthese electrolytes, those synthesized from mainly of lithium sulfide orsilicon sulfide are especially preferred because the vapor pressure ofeach component during synthesis is low and there is no need of sealingup the starting material for preventing evaporation loss of thecomponents. It suits to a large scale of preparation of theelectrolytes.

We have discussed the case of using indium or an indium-lithium alloy asthe material showing discontinuous change of potential consequent to theinsertion and extraction reaction of lithium ions into or from thelithium ion site in the crystal structure, but the following can becited as other examples of the materials showing the similar action.

First, the metallic elements are discussed.

In the insertion reaction, represented by the following formula (7), oflithium ions into the lithium ion site in the metal represented by theelemental symbol Me, in case Me is aluminum, there takes placediscontinuous change of potential from 0.35 V to around 0 V when x isabout 1.0. This indicates that the same effect as described above can beobtained by using aluminum in place of indium discussed above.

    Me+xLi.sup.- +xe.sup.- →Me-Li.sub.x                 (7)

Similar discontinuous change of potential occurs when x=2.0 in case Meis lead or bismuth, when x=2.5 in case Me is antimony, and when x=1.0 incase Me is gallium or tin. Such discontinuous change of potential alsooccurs in the lithium ion insertion reaction in case Me is silver,silicon, zinc, cadminum, arsenic or titanium, so that the same effectcan be obtained in case of using these materials.

In the case of a material showing plural discontinuous change ofpotential consequent to the lithium ion insertion/extraction reaction,the amount of the lithium ion inserted until discontinuous change ofpotential stated in claim 1 takes place may be of any amount that cancause any of said discontinuous changes of potential.

For instance, in case Me is tin in the formula (7), discontinuous changeof potential occurs not only when x=1.0 but also when x=2.3, 2.5, 2.6,3.5 or 4.4. Therefore, in the case of a solid-state secondary lithiumbattery using LiCoO₂ as positive electrode active material and tin asnegative electrode active material, the same effect as described abovecan be obtained by designing batteries in which the amount of thelithium ion inserted corresponding to the generation of the reactionwhen x=1.0 in the formula (9) at the negative electrode, or the amountof the lithium ion inserted corresponding to the generation of thereaction when x=2.3, 2.5, 2.6, 3.5 or 4.4 in the formula (8) at thenegative electrode is equal to or smaller than the amount of extractionof lithium ions corresponding to the generation of the reactionexpressed by formula (8) at the positive electrode.

    LiCoO.sub.2 →Li.sub.0.5 CoO.sub.2 +0.5Li.sup.+ +0.5e.sup.-(8)

    Sn+xLi.sup.+ +xe.sup.- →Sn-Li.sub.x                 (9)

In the case of an alloy composed of plural different metal elementsselected from indium, aluminum, lead, bismuth, antimony, gallium, tin,silver, silicon, zinc, cadminum, arsenic and titanium, they also showplural discontinuous changes of potential consequent to the insertionand extraction reaction of lithium ions. When these substances are usedas negative electrode active material, the amount of the lithium ioninserted until discontinuous change of potential described in claim 1occurs may be of any amount that can cause any of the prescribedpatterns of discontinuous changes of potential.

The alloys of these metals or their alloys with lithium may be used inplace of said pure metals or their alloys.

In case, for instance, an indium-lithium alloy represented by theformula In-Li₀.1 is used in place of metallic indium of theabove-mentioned solid-state secondary lithium battery using LiCoO₂ asthe positive electrode active material and indium as the negativeelectrode active material, the amount corresponding to y in thefollowing formula (10), namely the amount corresponding to y=0.9, is theamount of the lithium ion inserted until the occurrence of discontinuouschange of potential.

    In-Li.sub.0.1 +yLi.sup.+ +ye.sup.- →In-Li.sub.1.0   (10)

It is notable that especially when indium is used as a metal or as acomponent of a lithium alloy which shows discontinuous change ofpotential consequent to the insertion and extraction reaction of lithiumions into or from the lithium ion site in the crystal structure, it ispossible to obtain a solid-state secondary lithium battery which iscapable of operating with a large current because of the high diffusionrate of lithium ions in them. Therefore, in the present invention,indium or an indium-lithium alloy is preferably used as a metal or alloywhich shows discontinuous change of potential consequent to theinsertion/extraction reaction of lithium ions into or from the lithiumion site in the crystal structure.

It is also remarkable that in case aluminum is used as a metal or acomponent of a lithium alloy which shows discontinuous change ofpotential consequent to said lithium ion insertion and extractionreaction, it is possible to obtain a solid-state secondary lithiumbattery with high energy density because aluminum has a light atomicweight. Therefore, aluminum or an aluminum-lithium alloy is alsopreferred for use as a metal or alloy which shows discontinuous changeof potential consequent to said lithium ion insertion and extractionreaction.

Use of transition metal oxides, lithium transition metal oxides,transition metal sulfides and lithium transition metal sulfides as thematerial showing discontinuous change of potential consequent to saidlithium ion insertion and extraction reaction is discussed.

Of these materials, lithium titanium oxide (Li_(4/3) Ti_(5/d) O₄) havinga spinel structure shows an electrochemical lithium ion insertionreaction represented by the following formula (11) conductiveelectrolyte, and there takes place discontinuous change of potentialwhen x=1.0. Therefore, the same effect can be obtained in case of usingLi_(4/3) Ti_(5/3) O₄ in place of indium, etc., decribed above. LiTi₂ O₄is also capable of causing discontnuous change of potential at x=1.0 inthe formula (12).

    Li.sub.4/3 Ti.sub.5/3 O.sub.4 +xLi.sup.+ +xe.sup.- →Li.sub.4/3+x Ti.sub.5/3 O.sub.4                                        (11)

    LiTi.sub.2 O.sub.4 +xLi.sup.+ +xe.sup.- →Li.sub.1+x Ti.sub.2 O.sub.4(12)

Examples of the transition metal oxides or lithium transition metaloxides capable of causing such discontinuous change of potentialconsequent to said lithium ion insertion and extraction reactionsinclude TiO₂, MnO₂, WO₃, WO₂, V₂ O₅, etc., or Li_(x) Ti₂ O₂ Li_(x) MnO₂,Li_(x) WO₃, Li_(x) WO₂, Li_(x) V₂ O₅, etc., and use of these materialscan be also effective.

As the transition metal sulfide or lithium transition metal sulfide,there can be used, for example, TiS₂, MOS₂, NbS₂, WS₂ FeMo₆ S₈ and V₂ S₅or Li_(x) TiS₂, Li_(x) MnS₂, Li_(x) NbS₂, Li_(x) WS₂, Li_(x) Mo₆ S8 andLi_(x) V₂ S₅ as these materials show discontinuous change of potentialconsequent to the lithium ion insertion reaction.

The problems relating to the cycle characteristics of the batteryresulting from structural instability due to extraction of lithium ionsin a lithium transition metal compound described above become seriousespecially in a battery assembly in which a plurality of unit cells areconnected in series connection. However, by using an above-describedsolid-state secondary lithium battery at least as the unit cell havingthe smallest capacity in the set of the series-connected unit cells, itis possible to keep the unit cells at a depth of charging that allowsdevelopment of a desired cycle performance. Therefore, the presentinvention finds its particularly useful application to batteryassemblies.

Since said solid-state secondary lithium battery has no commonelectrolyte effect, a plurality of unit cells can be housed in a singleairtight container, allowing simple and compact design of the batterycase and manufacture of the uncostly battery assemblies.

Even in such a battery assembly, the terminal voltage shows adiscontinuous change in relation to the charged quantity of electricity,so that it is recommended to charge at a voltage below V₂ in a batteryassembly where there takes place discontinuous change of voltage from V₁to V₂.

When the term "potential" is used in the present specification, itrefers to not only equilibrium potential but also mixed potential in atwo-phase mixed state.

"Discontinuous change of potential" means that a sharp change ofpotential occurs with change of x in, for instance, the formula (6).That is, it means that when, for instance, the potential of In-Li_(x) inthe formula (6) is supposed to be E, the rate of change of E to changeof x (dE/dx) is maximized. Thus, the above term does not simply meandiscontinuity in a strictly mathematical sense, in other words, itsmeaning is not limited to the case where dE/dx diverges.

Said "sharp change of potential" is preferably the type of changedescribed below. In a solid-state secondary lithium battery, in case anelectrochemical reaction represented by the formula (13) is induced by acombination of a lithium transition metal oxide used as positiveelectrode active material, which causes the reversible electrochemicalinsertion and extraction reactions of lithium ions into or from thelithium ion site in the crystal structure, and a material used asnegative electrode material, which shows a discontinuous change ofpotential consequent to the lithium ion insertion and extractionreaction, there holds at least the relation of dE_(n) (x₀)/dx>dE_(p)(X₀)/dx, preferably dE_(n) (x₀)/dx>5 V, between the rate of change(dE_(p) (x)/dx) of positive electrode potential (E_(p) (x)) and the rateof change (dE_(n) (x)/dx) of negative electrodoe potential (dE_(n) (x))in the composition x=x₀ showing a discontinuous change of potential.

    Positive electrode reaction: LiMeO.sub.2 →Li.sub.1-x MeO.sub.2 +xLi.sup.+ +xe.sup.-

    Negative electrode reaction: Ox+xLi.sup.+ +xe.sup.- →Red(13)

When dE_(n) (x)/dx is smaller than the value that satisfies the aboverelation, the battery tends to be charged deeply due to change ofovervoltage or variation of charging voltage, making it hardly possibleto obtain a solid-state secondary lithium battery with excellent chargeand discharge characteristics.

Ox in the formula (13) represents a composition containing amother-phase metal such as In or Al in the case of In-Li_(x), Al-Li_(x)or like alloy, and containing one transition metal atom undergoingchange of valence shown in the formula (13) in the case of a transitionmetal oxide or transition metal sulfide. More specifically, itrepresents a composition such as Ti⁴⁺ S₂ or Li[Li_(1/3) Ti_(2/3) ]Ti⁴⁺O₄, in which valency is affixed only to the transition metal elementwhich is varied in formal valence.

It has been pointed out that the volumetric change of the electrodeactive material greatly affects the cycle characteristics of a solidbattery. For instance, in the reaction (formula (14)) of forming analuminum-lithium alloy by an electrochemical reaction of Al and Li⁺, thecrystal structures of Al and Al-Li are both of a cubic system, and theirlattice constants are 4.049A and 6.373A, respectively. Consequently, Aloccludes Li to form an Al-Li alloy, effecting approximately 3.9-foldexpansion of volume of the electrode active material.

    Al+Li.sup.+ +e.sup.- →Al-Li                         (14)

In contrast with such electrode active materials which are subject to alarge change of volume, there has been reported a composition Li_(4/3)Ti_(5/3) O₄ having a spinel structure has been reported as an electrodeactive material which is very small in change of volume onelectrochemical reaction with Li⁺. When a solid battery is constructedby using Li_(4/3) Ti_(5/3) O₄ as electrode active material, there isscarcely produced the influence of volumetric change such as mentionedabove, so that use of this material is expected to contribute to therealization of a solid battery having even more excellent cyclecharacteristics.

As a solid-state battery using Li_(4/3) Ti_(5/3) O₄ as electrode activematerial, there has been proposed the one using Li₀.33 La₀.56 TiO₃ assolid electrolyte (T. Brousse, P. Eragnaud, R. Marchand and D. M.Schleich: Extended Abstracts of 8th International Meeting on LithiumBatteries, 324 (1996), Nagoya; hereinafter referred to as Reference 1).However, since Li₀.33 La₀.56 is a solid electrolyte containingtransition metal atoms, it involves the problem that it is prone toelectrochemical reduction. For example, the solid electrolyte tends tobe reduced during charging at the interface of the negative electrode,consequently lessening the discharged capacity relative to the chargedcapacity as shown in FIG. 1 in Reference 1.

As viewed above, even when Li_(4/3) Ti_(5/3) O₄ is used as electrodeactive material, the charge/discharge efficiency is deteriorateddepending on the selection of solid electrolyte, causing instabilizationof the battery characteristics.

In contrast, an amorphous lithium ion conductive solid electrolytemainly composed of sulfides is electrochemically stable even at oraround the reducing potential of Li_(4/3) Ti_(5/3) O₄, and by combineduse of this solid electrolyte, it was realized for the first time toobtain high charge and discharge efficiency with Li_(4/3) Ti_(5/3) O₄and to produce a solid battery having excellent cycle characteristics.

The present invention is further illustrated by the following examples.All of the operations described in the following Examples were carriedout under a dry argon atmosphere.

EXAMPLES Example 1

A solid-state secondary lithium battery was made by using lithium cobaltoxide (LiCoO₂) as a lithium transition metal oxide showing thereversible electrochemical insertion and extraction reactions of lithiumions into or from the lithium ion site in the crystal structure and usedas positive electrode active material, indium as the material showingdiscontinuous change of potential consequent to the insertion andextraction reactions of lithion ions into or from the lithium ion sitein the crystal structure, and an amorphous solid electrolyte (0.01Li₃PO₄ --0.63Li₂ S--0.36SiS₂) as a lithium ion conductive solidelectrolyte, and the properties of the produced battery were evaluated.Details of the process are described below.

First, a lithium ion conductive solid electrolyte was synthesized in thefollowing way.

Lithium sulfide, silicon sulfide and lithium phosphate were weighed andmixed in a ratio of 63:36:1. This mixture was filled in a glassycarbon-made crucible and melted at 1,000° C. in an argon gas stream for2 hours. The melt was rapidly cooled with twin rolls to prepare alithium ion conductive solid electrolyte.

Then LiCoO₂ was synthesized by weighing and mixing cobalt oxide (Co₃ O₄)and lithium carbonate (Li₂ CO₃) so that the mixture would have a Co/Liratio of 1 and calcining the mixture at 900° C. in the atmosphere.

Using the thus obtained lithium ion conductive solid electrolyte, apositive electrode active material and a foil of metallic indium, asolid-state secondary lithium battery was constructed in the followingway.

A sectional view of the solid-state secondary lithium battery Aaccording to the instant embodiment of the invention is shown in FIG. 3.In the drawing, reference numeral 1 designates positive electrodeconstituted by using weighed 300 mg of a positive electrode materialprepared by mixing said LiCoO₂ and the pulverized solid electrolyte in aweight ratio of 6:4. Numeral 2 refers to a lithium ion conductive solidelectrolyte layer which was pressure molded integrally with a metallicindium foil 3 weighing 95 mg, which constitutes the negative electrode.The stacked molded pellets were put into a stainless steel-made batterycontainer 4 and the container was closed airtightly by a stainlesssteel-made cover 6 with the aid of an insulating gasket 5.

In this solid-state secondary lithium battery A, the maximum (amount of)extraction of lithium ions within the range where the reversibleinsertion/extaction reaction of lithium ions into or from the lithiumcobalt oxide take place is 24.6 mAh, which corresponds to the reactionof the formula (8), while the lithium ion inserted until discontinuouschange of potential of indium in the course of the insertion/extractionreaction of lithium ions into or from indium occurs is 22.2 mAh, whichcorresponds to the reaction of the formula (3). In this battery,therefore, the amount of the lithium ion inserted until the occurrenceof discontinuous change of potential of the material showingdiscontinuous change of potential consequent to the lithium ionionsertion and extraction reaction is smaller than the maximumextraction of lithium ions which causes reversible reaction range of thelithium transition metal oxide.

By way of comparison, a solid-state secondary lithium battery B wasconstructed in the same way as described above except that the indiumfoil weighed 190 mm. In this battery, the amount of lithium ion inserteduntil discontinuous change of indium potential occurred corresponded to44.4 mAh, and the amount of lithium ion inserted until potential of thematerial showing discontinuous change of potential consequent to thelithium ion insertion/extraction reaction changed discontinously wasgreater than the maximum amount of extraction of lithium ions of thereversible reaction range of the lithium transition metal oxide.

The thus obtained solid-state secondary lithium batteries were chargedby applying a current of 500 μA. FIG. 4 shows the first charging curvesof these batteries. In the case of the solid-state secondary lithiumbattery A, the voltage 3.85 V at which discontinuous change of thecharging curve occurred was made the termination voltage of charging. Inthe case of the solid-state secondary lithium battery, the terminalvoltage on charging of 24.6 mAh was made the charge termination voltage.

The charge termination voltage was decided in this way. Further, withthe discharge termination voltage being set at 2.0 V, a charge anddischarge cycle test with a charge/discharge current of 500 μA wasconducted. FIG. 5 shows the discharged quantity of electricity in eachcycle.

In the solid-state secondary lithium battery A according to the presentinvention, there was observed little change of the discharged quantityof electricity with the charge and discharge cycle, but in thesolid-state lithium seconary battery B made for the purpose ofcomparison, there was seen a drop of discharge capacity with the chargeand discharge cycle.

Then, influence of drift of charge voltage on the charge and dischargecycle behavior was examined in the following way.

For the charger, there was used a power source capable of generating avoltage equal to a combination of a DC voltage of 3.75 V and an ACvoltage with an amplitude of 0.1 V and a frequency of 1 mHz. Each of thesolid-state secondary lithium batteries was subjected to a charge anddischarge cycle test in which each battery was first charged for 50hours by said charger and then discharged to 2.0 V with a constantcurrent of 500 μA, with the discharge capacity in each cycle beingrecorded. The results are shown in FIG. 6.

In the solid-state secondary lithium battery A, there was observedlittle change in discharge capacity with the charge and discharge cycle,while in the solid-state secondary lithium battery B, discharge capacitywas low from the beginning and a further drop of discharge capacity wasseen as the charge and discharge cycle went on.

It has thus been confirmed that a solid-state secondary lithium batterywith excellent charge and discharge cycle characteristics can beobtained according to the present invention.

Example 2

A solid-state secondary lithium battery was constructed in the same wayas in Example 1 except that aluminum was used in place of indium as thematerial showing discontinuous change of potential consequent to theinsertion/extraction reaction of lithium ions into or from the lithiumion site in the crystal structure, and the properties of this batterywere evaluated. The process is detailed below.

A lithium ion conductive solid electrolyte and a lithium cobalt oxidewere synthesized, weighed and mixed in the same way as in Example 1 toform the positive electrode of the battery.

A 1:5 (by weight) mixture of a pulverized solid electrolyte andpulverized metallic aluminum was used as negative electrode material. 25mg of this negative electrode material was weighed and worked to formthe negative electrode of the battery.

Using the thus obtained positive and negative electrodes, a solid-statesecondary lithium battery C was made in the same way as in Example 1. Inthis battery, the maximum amount of extraction of lithium ions withinthe range, where the reversible insertion and extraction reactions oflithium ions into or from the lithium cobalt oxide take place, was 24.6mAh, the same as in Example 1, and the amount of lithium ions inserteduntil discontinuous change of potential of aluminum in theinsertion/extraction reactions of lithium ions into or from aluminumtook place was 20.7 mAh corresponding to the reaction of the formula(4). In this battery, therefore, the amount of insertion of lithium ionsuntil potential of the material showing discontinuous change ofpotential consequent to the lithium ion insertion/extraction reactionchanges discontinuously is smaller than the maximum amount of extractionof lithium ions of the reversible insertion/extraction reactions oflithium ions into or from the lithium transition metal oxide.

Then, by way of comparison, a solid-state secondary lithium battery Dwas made in the same way as described above except that the negativeelectrode weighed 50 mg. In this battery, the amount of lithium ioninserted until discontinuous change of aluminum potential took place was41.4 mAh, and the amount of lithium ions inserted until potential of thematerial showing discontinuous change of potential consequent to theamount of lithium ion insertion and extraction reaction changeddiscontinously was greater than the maximum extraction amount of lithiumions of reversible reaction range.

The thus constructed solid-state secondary lithium batteries weresubjected to the same charge and discharge test as conducted inExample 1. In the case of the solid-state secondary lithium battery Caccording to the present invention, the voltage which causeddiscontinuous change of the charging curve was set as the chargetermination voltage, while in the comparative solid-state secondarylithium battery D, the terminal voltage on charging of 24.6 mAh was setas the charge termination voltage.

As a result, in the solid-state secondary lithium battery C according tothe present invention, there was observed little change of dischargedquantity of electricity with the charge and discharge cycle, but in thecomparative solid-state secondary lithium battery D, there was seen adrop of discharge capacity with the charge and discharge cycle.

Then, influence of draft of charge voltage on the charge and dischargecycle behavior was examined in the same way as in Example 1 except thatthe DC voltage of the charger was set at 4.05 V.

As a result, in the solid-state secondary lithium battery C, there wasobserved substantially no drop of discharge capacity with the charge anddischarge cycle, while in the solid-state secondary lithium battery D,discharge capacity was low from the beginning and a further drop ofdischarge capacity was seen as the charge and discharge cycle went on.

It has thus been confirmed that a solid-state secondary lithium batterywith excellent charge and discharge cycle characteristics can beobtained according to the present invention.

Examples 3-13

The solid-state secondary lithium batteries were constructed in the sameway as in Example 2 except that the materials shown in Table 1 wereused, in place of aluminum, as the material showing discontinuous changeof potential consequent to the insertion/extraction reactions of lithiumions into or from the lithium ion site in the crystal structure, and thecharge and discharge cycle behavior of these batteries under thecondition of constant current application and influence of drift ofcharge voltage on the charge and discharge cycle were examined in thesame way as in Example 2. The negative electrodes of these batterieswere prepared by using the weighed amounts (shown in Table 1) of the 5:1(by weight) mixtures of said metals or alloys and a pulverized solidelectrolyte. Also shown in Table 1 are the maximum amount of extractionQ1 of lithium ions which causes the reversible insertion/extractionreactions of lithium ions into or from the lithium cobalt oxide in eachbattery and the insertion amount Q2 of lithium ions until ofdiscontinuous change of potential of the negative electrode took placein the course of the insertion/extraction reactions of lithium ions intoor from the negative electrode of each battery. In these batteries,Q2<Q1.

There was observed no drop of discharge capacity with the charge anddischarge cycle, and it has been confirmed that a solid-state secondarylithium battery with excellent charge and discharge cyclecharacteristics can be obtained according to the present invention.

                  TABLE 1                                                         ______________________________________                                                          Negative                                                    Negative electrode                                                                              electrode Q1                                                active material   weight (mg)                                                                             (mAh)   Q2 (mAh)                                  ______________________________________                                        Example 3                                                                             Lead          100       24.6  21.6                                    Example 4                                                                             Bismuth       100       24.6  21.3                                    Example 5                                                                             Antimony      40        24.6  18.8                                    Example 6                                                                             Gallium-lithium alloy                                                                       60        24.6  18.8                                            Ga.sub.1.0 --Li.sub.0.21                                              Example 7                                                                             Tin           45        24.6  21.2                                    Example 8                                                                             Silver        50        24.6  20.7                                    Example 9                                                                             Silicon       5         24.6  15.9                                    Example 10                                                                            Zinc          60        24.6  20.5                                    Example 11                                                                            Cadminum      100       24.6  19.9                                    Example 12                                                                            Arsenic       20        24.6  17.9                                    Example 13                                                                            Titanium      40        24.6  18.7                                    ______________________________________                                    

Example 14

A solid-state secondary lithium battery was constructed in the same wayas in Example 1 except that a lead-indium alloy was used in place ofindium as the material showing discontinuous change of potentialconsequent to the insertion/extraction reaction of lithium ions into orfrom the lithium ion site in the crystal structure, and the propertiesof the obtained battery were evaluated. The process is detailed below.

The lead-indium (Pb-In) alloy was prepared by melting a 1:1 (by mole)mixture of metallic lead and metallic indium at 800° C. in an argonstream.

A lithium ion conductive solid electrolyte and a lithium cobalt oxidewere synthesized, weighed and mixed in the same way as in Example 1 toform the positve electrode of the battery.

A 1:5 (by weight) mixture of a pulverized solid electrolyte and apulverized lead-indium alloy was used as negative electrode material,and 100 mg of this negative electrode material was weighed and workedinto the negative electrode of the battery.

In the insertion reaction of lithium ions into this negative electrode,the amount of insertion of lithium ions causing discontinuous change ofpotential of the negative electrode was 6.94 mAh corresponding to thereaction of the formula (15):

    Pb-In+Li.sup.+ +e.sup.- →Pb-In-Li                   (15)

and 20.18 mAh corresponding to the reaction of the formula (16):

    Pb-In+3Li.sup.+ +3e.sup.- →Pb-In-Li.sub.3           (16)

A 6:4 (by weight) mixture of LiCoO₂ obtained in Example 1 and apulverized solid electrolyte was used as positive electrode material. 91mg of this positive electrode material was weighed and worked, and asolid-state secondary lithium battery E was constituted in the same wayas in Example 1. Another solid-state secondary lithium battery F wasmade likewise except for using 304 mg of said positive electrodematerial.

In these solid-state secondary lithium batteries, the maximum amount ofextraction of lithium ions within the rarge of the reversibleinsertion/extraction reactions of lithium ions into or from the lithiumcobalt oxide was 7.5 mAh in the case of the battery E corresponding tothe reaction of the formula (8) and 25 mAh in the case of the battery F,and was greater than the amount of insertion of lithium ions causingdiscontinuous change of potential of the negative electrodecorresponding to the reaction of the formula (15) or (16).

In the thus constructed solid-state secondary lithium batteries,influence of drift of charge voltage on the charge and discharge cyclebehaviors was examined in the same way as in Example 1.

There was observed no drop of discharge capaicity with the charge anddischarge cycle, and it has been confirmed that a solid-state secondarylithium battery with excellent charge and discharge cyclecharacteristics can be obtained according to the present invention.

Example 15

A solid-state secondary lithium battery was made in the same way as inExample 1 except that a lithium titanium oxide was used in place ofindium as the material showing discontinuous change of potentialconsequent to the insertion/extraction reaction of lithium ions into orfrom the lithium ion site in the crystal structure, and the propertiesof the battery were evaluated. The process is detailed below.

First, lithium titanium oxide was synthesized in the following way.

Lithium hydroxide (LiOH) and titanium oxide (TiO₂) were used as startingmaterials. The starting materials were weighed and mixed so that themixture would have a Li:Ti ratio of 4:5. This mixture was pressuremolded into pellets and heated at 900° C. in the air for 20 hours toobtain lithium titanium oxide of the formula Li_(4/3) Ti_(5/3) O₄.

A lithium ion conductive solid electrolyte and a lithium cobalt oxidewere synthesized, weighed and mixed in the same way as in Example 1 toform the positive electrode of the battery.

A 2:3 (by weight) mixture of a pulverized solid electrolyte and thelithium titanium oxide obtained in the manner described above was usedas negative electrode material. 209 mg of this negative electrodematerial was weighed and worked into the negative electrode of thebattery.

Using the thus prepared positive and negative electrodes, a solid-statesecondary lithium battery G was constructed in the same way as inExample 1. In this battery G, the maximum amount of extraction oflithium ions within the reversible insertion/extraction reaction rangeof lithium ions into or from the lithium cobalt oxide was 24.6 mAh, thesame as in Example 1, while the insertion amount of lithium ions untildiscontinuous change of potential of the lithium titanium oxide tookplace in the insertion/extraction reactions of lithium ions into or fromthe lithium titanium oxide was 22 mAh corresponding to the reaction ofthe formula (17):

    Li.sub.4/3 Ti.sub.5/3 O.sub.4 +Li.sup.+ +e.sup.- →Li.sub.7/3 Ti.sub.5/3 O.sub.4                                        (17)

In this battery, therefore, the amount of insertion of lithium ionsuntil potential of the material showing discontinuous change ofpotential consequent to the lithium ion insertion/extraction reactionchanges discontinuously is smaller than the maximum amount of extractionof lithium ions within the reversible insertion/extraction of lithiumions into or from the lithium transition metal oxide.

Then, by way of comparison, a solid-state secondary lithium battery Hwas made in the same way as described above except that the negativeelectrode weighed 518 mg. In this battery, the amount of lithium ionsinserted until discontinous change of potential of the lithium titaniumoxide took place was 44 mAh, and the amount of lithium ions inserteduntil potential of the material showing discontinuous change ofpotential consequent to the lithium ion insertion/extraction reactionchanged discontinuously was greater than the maximum amount ofextraction of lithium ions which causes reversible insertion/extractionreaction of lithium ions into or from the lithium transition metaloxide.

The thus constructed solid-state secondary lithium batteries weresubjected to the same charge and discharge test as conducted inExample 1. In the case of the solid-state secondary lithium battery Gaccording to the present invention, the voltage at discontinuous changeof the charging curve was set as the charge termination voltage, whilein the case of the comparative solid-state secondary lithium battery H,the terminal voltage on charging of 24.6 mAh was set as the chargetermination voltage.

As a result, in the solid-state secondary lithium battery G according tothe present invention, there was observed little change in dischargedquantity of electricity with the charge and discharge cycle, while inthe comparative solid-state secondary lithium battery H, there was seena drop of discharge capacity with the charge and discharge cycle.

Next, the charge and discharge cycle was examined in the same way as inExample 1 except that the DC voltage of the charger was set at 3.25 V.

As a result, in the solid-state secondary lithium battery G, there wasobserved substantially no drop of discharge capacity with the charge anddischarge cycle, while in the solid-state secondary lithium battery H,discharge capacity was low from the beginning and a further drop ofdischarge capacity with the charge and discharge cycle was seen.

It has thus been confirmed that a whole-solid secondary lithium batterywith excellent charge and discharge cycle characteristics can beobtained according to the present invention.

Example 16

A solid-state secondary lithium battery was made in the same way as inExample 1 except that titanium disulfide was used in place of indium asthe material showing discontinuous change of potential consequent to theinsertion/extraction reaction of lithium ions into or from the lithiumion site in the crystal structure, and the properties of the batterywere evaluated. The process is detailed below.

Titanium disulfide (TiS₂) was synthesized by the CVD method using sulfurand metallic titanium as starting materials.

A lithium ion conductive solid electrolyte and a lithium cobalt oxidewere synthesized, weighed and mixed in the same way as in Example 1 toform the positive electrode of the battery.

A 2:3 (by weight) mixture of a pulverized solid electrolyte and thetitanium disulfide obtained in the manner described above was used asnegative electrode material. 153 mg of this negative electrode materialwas weighed and worked into the negative electrode of the battery.

Using the thus prepared positive and negative electrodes, a solid-statesecondary lithium battery I was made in the same way as in Example 1. Inthis whole-solid secondary lithium battery, the maximum amount oflithium ion extraction within the range where the reversibleinsertion/extraction reactions of lithium ions into or from the lithiumcobalt oxide are reversible was 24.6 mAh, the same as in Example 1,while the amount of lithium ions inserted until discontinuous change ofpotential of titanium disulfide took place in the insertion/extractionreaction of lithium ions into or from titanium disulfide was 22 mAhcorresponding to the reaction of the formula (18):

    TiS.sub.2 +Li.sup.+ +e.sup.- →LiTiS.sub.2           (18)

In this battery, therefore, the amount of lithium ion inserted untilpotential of the material showing discontinuous change of potentialconsequent to the lithium ion insertion/extraction reaction changesdiscontinuously is smaller than the maximum amount of extraction oflithium ions within the casuses reversible insertion/extraction range oflithium ions into or from the lithium transition metal oxide.

Then, by way of comparison, a solid-state secondary lithium battery Jwas made in the same way as described above except that the negativeelectrode weighed 306 mg. In this battery, the amount of lithium ionsinserted until discontinuous change of potential of lithium titaniumoxide took place was 44 mAh, and hence the amount of lithium ionsinserted until potential of the material showing discontinuous change ofpotential consequent to the lithium ion insertion/extraction reactionchange discontinuously was greater than the maximum amount of extractionof lithium ions within the rarge where the insertion/extractionreactions of lithium ions into or from the lithium transition metaloxide are reversible.

These solid-state secondary lithium batteries were subjected to the samecharge and discharge test as conducted in Example 1. In the case of thebattery I according to the present invention, the voltage indiscontinuous change of the charging curve was set as the chargetermination voltage, while in the case of the comparative battery J, theterminal voltage on charging of 24.6 mAh was set as the chargetermination voltage.

As a result, in the solid-state secondary lithium battery I according tothe present invention, there was observed substantially no change indischarged quantity of electricity with the charge and discharge cycle,while in the comparative solid-state secondary lithium battery J, therewas seen a drop of discharge capacity with the charge and dischargecycle.

Then, the charge and discharge cycle was examined in the same way as inExample 1 except that the DC voltage of the charger was set at 2.5 V.

As a result, in the solid-state secondary lithium battery I, there wasobserved substantially no drop of discharge capacity with the charge anddischarge cycle, while in the solid-state secondary lithium battery J,discharge capacity was small from the beginning and there was seen afurther drop of discharge capacity with the charge and discharge cycle.

It has thus been confirmed that a solid-state secondary lithium batterywith excellent charge and discharge cycle characteristics can beobtained according to the present invention.

Example 17

A solid-state secondary lithium battery was made in the same way as inExample 1 except that a lithium nickel oxide (LiNiO₂) was used in placeof the lithium cobalt oxide (LiCoO₂) as positive electrode activematerial. The process is detailed below.

First, LiNiO₂ was synthesized by mixing nickel oxide (NiO) and lithiumhydroxide and heating the mixture at 800° C. in the atmosphere.

The thus obtained LiNiO₂ was pulverized and mixed with the solidelectrolyte obtained in Example 1 in a weight ratio of 3:2 to prepare apositive electrode material. A solid-state secondary lithium battery Kwas made in the same way as in Example 1 except that 300 mg of saidpositive electrode material was weighed and worked into positiveelectrode.

In this solid-state secondary lithium battery, the maximum amount ofextraction of lithium ions within the range where theinsertion/extraction reaction of lithium ions into or from the lithiumcobalt oxide is reversible is 24.7 mAh corresponding to the reaction ofthe formula (19):

    LiNiO.sub.2 →Li.sub.0.5 NiO.sub.2 +0.5Li.sup.+ +0.5e.sup.-(19)

The amount of lithium ions inserted until discontinuous change ofpotential of indium takes place in the course of theinsertion/extraction reaction of lithium ions into or from indium is22.2 mAh corresponding to the reaction of the formula (3) as inExample 1. In this battery, therefore, the lithium ions inserted untilpotential of the material showing discontinuous change of potential inthe course of the lithium ion insertion/extraction reaction changesdiscontinuously is smaller than the maximum extraction of lithium ionsthat does not cause irreversible insertion and extraction of lithiumions into or from the lithium transition metal oxide.

Then, by way of comparison, a solid-state secondary lithium battery Lwas made in the same way as described above except that the indium foilweighed 190 mg. In this battery, the amount of lithium ion inserteduntil discontinuous change of potential of indium takes place is 44.4mAh, and the amount of lithium ions inserted until potential of thematerial showing discontinuous change of potential consequent to thelithium ion insertion/extraction reaction changes discontinuously isgreater than the maximum extraction of lithium ions within thereversible insertion/extraction of lithium ions into or from the lithiumtransition metal oxide.

The thus composed solid-state secondary lithium battery was subjected tothe same charge and discharge test as conducted in Example 1. In thecase of the battery K according to the present invention, the voltagewhen the charging curve was discontinuously charged was set as thecharge termination voltage, while in the comparative battery L, theterminal voltage on charging of 24.6 mAh was set as the chargetermination voltage.

As a result, in the battery K of the present invention, there wasobserved substantially no change in discharged quantity of electricitywith the charge and discharge cycle, while in the comparative battery L,there was seen a drop of discharge capacity with the charge anddischarge cycle.

Next, influence of time-dependent variation of the charging voltage onthe cycle behavior was examined in the same way as in Example 1 exceptthat the DC voltage of the charger was set at 3.5 V.

As a result, in the battery K, there was observed substantially no dropof discharge capacity with the charge and discharge cycle, while in thebattery L, discharge capacity was small from the beginning and a furtherdrop of discharge capacity was seen with the charge and discharge cycle.

It has thus been confirmed that a solid-state secondary lithium batterywith excellent charge and discharge cycle characteristics can beobtained according to the present invention.

Example 18

A solid-state secondary lithium battery was made in the same way as inExample 1 except that a lithium manganese oxide (LiMn₂ O₄) was used inplace of the lithium cobalt oxide (LiCoO₂) as positive electrodematerial. The process is detailed below.

LiMn₂ O₄ was synthesized by mixing lithium carbonate (Li₂ CO₃) andmanganese acetate (Mn(CH₃ COO)₂) and heating the mixture at 750° C. inthe atmosphere.

The thus obtained LiMn₂ O₄ was pulverized and mixed with the solidelectrolyte obtained in Example 1 and acetylene black (electronconductive material) in a weight ratio of 3:1.9:0.1 to prepare apositive electrode material. A solid-state secondary lithium battery Mwas made in the same way as in Example 1 except that 432 mg of saidpositive electrode material was weighed and worked into positiveelectrode.

In this solid-state secondary lithium battery M, the maximum amount ofextraction of lithium ions in the range of the reversibleinsertion/extraction reactions of lithium ions into or from the lithiummanganese oxide was 25.0 mAh corresponding to the reaction of theformula (20):

    LiMn.sub.2 O.sub.4 →Li.sub.0.35 Mn.sub.2 O.sub.4 +0.65Li.sup.+ +0.65e.sup.-                                              (20)

while the amount of lithium ion inserted until discontinuous change ofpotential of indium took place in the course of the insertion/extractionreaction of lithium ions into or from indium was 22.2 mAh correspondingto the reaction of the formula (3) as in Example 1. In this battery,therefore, the lithium ions inserted until potential of the materialshowing discontinuous change of potential consequent to with the lithiumion insertion/extraction reaction changes discontinuously is smallerthan the maximum extraction of lithium ions within the reversibleinsertion/extraction of lithium ions into or from the lithium transitionmetal oxide.

Then, by way of comparison, a solid-state secondary lithium battery Nwas made in the same way as described above except that the indium foilweighed 190 mg. In this battery, the amount of lithium ions inserteduntil discontinuous change of potential of indium took place was 44.4mAh, and the lithium ion inserted until potential of the materialshowing discontinuous change of potential consequent to the lithium ioninsertion/extraction reaction changes discontinuously is greater thanthe maximum extraction of lithium ions within the reversibleinsertion/extraction of lithium ions into or from the lithium transitionmetal oxide.

The thus composed solid-state secondary lithium batteries were subjectedto the same charge and discharge test as conducted in Example 1. In thecase of the battery M according to the present invention, the voltage atdiscontinuous change of the charging curve was set as the chargetermination voltage, while in the comparative battery N, the terminalvoltage on charging of 24.6 mAh was set as the charge terminationvoltage.

As a result, in the battery M according to the present invention, therewas observed substantially no change in discharged quantity ofelectricity with the charge and discharge cycle, while in thecomparative battery N, there was seen a drop of discharge capaicty withthe charge and discharge cycle.

Then, the charge and discharge cycle behavior on time-dependentvariation of charging voltage was examined the same way as in Example 1except that the DC voltage of the charger was set at 3.65 V.

As a result, in the battery M, there was observed substantially nodegradation of discharge capacity with the charge and discharge cycle,while in the battery N, discharge capacity was low from the beginningand there was observed a further drop of discharge capacity with thecharge and discharge cycle.

It has thus been confirmed that a solid-state secondary lithium batterywith excellent charge and discharge cycle characteristics can beobtained according to the present invention.

Example 19

A solid-state lithium battery was made in the same way as in Example 1except that an amorphous solid electrolyte (0.05Li₂ O--0.57Li₂S--0.38SiS₂) was used in place of 0.01Li₃ PO₄ --0.63Li₂ S--0.36SiS₂obtained in Example 1 as the lithium ion conductive solid electrolyte,and the properties of the battery were evaluated. The process isdetailed below.

Lithium sulfide, silicon sulfide and lithium oxide were weighed andmixed in a molar ratio of 57:38:5, and the mixture was filled in aglassy carbon-made crucible and melted at 1,000° C. in a nitrogen gasstream for 2 hours. The melt was ultra-rapidly cooled by a twin roll inthe same way as in Example 1 to obtain a lithium ion conductive solidelectrolyte.

Using this lithium ion conductive solid electrolyte, there were made asolid-state secondary lithium battery according to the present inventionand a comparative solid-state secondary lithium battery, both using thesame amount of indium foil as in Example 1, and these batteries weresubjected to the same charge and discharge test as conducted in Example1.

As a result, in the battery according to the present invention, therewas observed substantially no change in discharged capacity with thecharge and discharge cycle, while in the comparative battery, there wasseen a decrease of discharge capacity with the charge and dischargecycle.

Then, the charge and discharge cycle behavior on time-dependentvariation of charging voltage was examined in the same way as in Example1.

As a result, the battery of the present invention showed substantiallyno drop of discharge capacity with the charge and discharge cycle, whilethe comparative battery was small in discharge capacity from thebeginning and showed a further drop of discharge capacity with thecharge and discharge cycle.

It has thus been confirmed that a solid-state secondary lithium batterywith excellent charge and discharge cycle characteristics can beobtained according to the present invention.

Example 20

A battery assembly was constructed by using a lithium cobalt oxide(LiCoO₂) as the lithium transition metal oxide which causes thereversible electrochemical insertion/extraction reactions of lithiumions into or from the lithium ion site in the crystal structure and isused as positive electrode active material as in Example 1, indium asthe material showing discontinuous change of potential consequent toinsertion/extraction reaction of lithium ions, and an amorphous solidelectrolyte (0.01Li₃ PO₄ --0.63Li₂ S--0.36SiS₂) as the lithium ionconductive solid electrolyte, and the properties of this battery wereevaluated. The process is detailed below.

A sectional view of the battery assembly according to the instantembodiment of the invention is shown in FIG. 7, and the compositions ofthe unit cells are shown in Table 2.

                                      TABLE 2                                     __________________________________________________________________________    Unit cell (i)          Unit cell (ii)                                              Positive                                                                           Negative     Positive                                                                           Negative                                          Battery                                                                            electrode                                                                          electrode                                                                          Q1  Q2  electrode                                                                          electrode                                                                          Q1  Q2                                       assembly                                                                           (mg) (mg) (mAh)                                                                             (mAh)                                                                             (mg) (mg) (mAh)                                                                             (mAh)                                    __________________________________________________________________________    S1   150  50   12.3                                                                              11.7                                                                              150  50   12.3                                                                              11.7                                     S2   150  50   12.3                                                                              11.7                                                                              150  70   12.3                                                                              16.3                                     S3   150  50   12.3                                                                              11.7                                                                              120  50   9.9 11.7                                     S4   120  50   9.9 11.7                                                                              120  50   9.9 11.7                                     __________________________________________________________________________

In FIG. 7, reference numerals 7 and 8 indicate the unit cell (i) and theunit cell (ii), respectively, whose compositions are shown in Table 2.These two unit cells were connected in series by a connector 9 andplaced in a stainless battery container 10, the battery container beingclosed airtightly by a cover 12 with the aid of an insulating gasket 11.Battery assemblies S1-S4 were made in this way.

In Table 2 are shown the weights of the positive and negativeelectrodes, the amount of maximum extraction Q1 within the range wherethe insertion/extraction reactions of lithium ions into or from thelithium cobalt oxide are reversible in each unit cell, namely thequantity of electricity corresponding to the reaction of the formula(8), and the amount of lithium ion inserted Q2 until discontinuouschange of potential of indium took place in the course of theinsertion/extraction reaction of lithium ions into or from indium,namely the quantity of electricity corresponding to the reaction of theformula (3).

In the battery assembly S1, both of the unit cells (i) and (ii) are socomposed that the lithium ion inserted Q2 until potential of thematerial showing discontinuous change of potential consequent to thelithium ion insertion/extraction reaction changes discontinuously willbe smaller than the maximum amount of extraction Q2 of lithium ions inthe reversible insertion/extraction of lithium ions into or from thelithium tansition metal oxide, which conforms to the conditions of thepresent invention for making a battery assembly. Also, in the batteryassembly S2, the relation of Q1>Q2 holds in the unit cell (i) which issmaller in capacity than the other unit cell (ii). Thus, these batteriesconform to the specified conditions of the present invention.

In the battery assembly S3, on the other hand, Q1<Q2 in the unit cell(ii) having the smaller capacity, and in the battery assembly S4, Q1<Q2in both of the unit cells (i) and (ii), so that these battery assembliesdo not meet the conditions of the present invention.

These battery assemblies were subjected to the following charge anddischarge cycle test.

The battery assemblies were charged by applying a constant current of250 μA. Set as the battery charge termination voltage was set to thevoltage at which caused either (1) flow of a quantity of chargedelectricity corresponding to the reaction of letting the lithium cobaltoxide in the positive electrode of either of the unit cells has thecomposition of Li₀.5 CoO₂ or (2) the charging curve changeddiscontinuously during charging.

The discharge current was 250 μA and the discharge termination voltagewas 4.5 V.

In the charge and discharge cycle test conducted on these batteryassemblies under the above charge and discharge conditions, the batteryassemblies S1 and S2 showed substantially no change of charge anddischarge behavior with the charge and discharge cycle, while thebattery assemblies S3 and S4 showed a decrease in charge and dischargecapacity with the charge and discharge cycle.

Then, influence of voltage drift during charging on the charge anddischarge cycle behavior was examined in the following way.

For the charger, there was used a power source capable of generating avoltage equal to the combination of a voltage corresponding to thecharge termination voltage decided in said constant-current charge anddischarge cycle test and an AC voltage with an amplitude of 0.1 V and afrequency of 1 mHz. Each battery assembly was charged by this chargerfor 50 hours and then subjected to a charge and discharge cycle test inwhich each battery assembly was discharged down to 4.5 V with a constantcurrent of 250 μA.

As a result, the battery assemblies S1 and S2 showed substantially nodrop of discharge capacity with the charge and discharge cycle, whilethe battery assemblies S3 and S4 were small in discharge capacity fromthe beginning and showed a further drop of discharge capacity with thecharge and discharge cycle.

It has thus been confirmed that a battery assembly with excellent chargeand discharge cycle characteristics can be obtained according to thepresent invention.

Example 21

The influence of volumetric change of the electrode active material onthe charge and discharge cycle characteristics was examined. In otherexamples, a solid-state secondary lithium battery was constituted byencapsulating the molded solid-state secondary lithium battery pelletsin a coin battery case, but in the present example the solid-statesecondary lithium battery pellets were encapsulated in a laminated filmto constitute a battery and its properties were examined.

Using Li_(4/3) Ti_(5/3) O₄ as the negative electrode active material andLiCoO₂ as the positive electrode active material, there were molded thesolid-state secondary lithium battery pellets same as those of thesolid-state secondary lithium battery G of Example 15. A stainlesssteel-made mesh with steel-made lead wires spot welded to it was pressbonded to each of the positive and negative electrodes of the moldedsolid-state secondary lithium battery pellets as current collectors andlead terminal. These were encapsulated in a film made by laminating apolyethylene sheet on a stainless foil to form a solid-state secondarylithium battery O.

Then, by way of comparison, the solid-state secondary lithium batterypellets same as those of the solid-state secondary lithium battery C ofExample 2 were molded by using Al as negative electrode active materialand LiCoO₂ as positive electrode active material, and a solid-statesecondary lithium battery P was made therewith in the same way as in thecase of the solid-state secondary lithium battery O.

These solid-state secondary lithium batteries were subjected to the samecharge and discharge test as conducted on the battery G of Example 15and the battery C of Example 2.

As a result, the solid-state secondary lithium battery O according tothe present invention showed substantially no change in dischargedquantity of electricity with the charge and discharge cycle, while thesolid-state secondary lithium battery P made for comparison was small indischarge capacity from the beginning and showed a further drop ofdischarge capacity with the charge and discharge cycle.

The unsatisfactory properties of the comparative solid-state secondarylithium battery P may be accounted for by the fact that because thebattery case was replaced by a laminated film, the pressure given fromthe case to the battery pellets contained in the case was reduced,making the battery composing elements more liable to become weakened inbonding therebetween with a volumetric change of the active material.

It has thus been confirmed that a solid-state secondary lithium batterywith excellent charge and discharge cycle characteristics can beobtained according to the present invention.

In the foregoing Examples of the present invention, explanations wereconcentrated on the solid-state secondary lithium batteries made byusing indium, aluminum, lead-indium alloy, lithium titanium oxide ortitanium disulfide as the material showing discontinuous change ofpotential consequent to the insertion/extraction reaction of lithiumions into or from the lithium ion site in the crystal structure, but itgoes without saying that the similar effect can be obtained when usingother metals or alloys, transition metal oxides such as MnO₂, WO₃, WO₂and V₂ O₅, transition metal sulfides such as MoS₂, NbS₂ and V₂ S₄, orcomposite oxides as said material. Thus, the present invention is notlimited to the above-described embodiments using the mentionedsubstances as the material showing discontinuous change of potentialconsequent to the insertion/extraction reaction of lithium ions into orfrom the lithium ion site in the crystal structure.

Also, in the above Examples of the present invention, there were onlyshown the embodiments comprising the positive electrodes made by usinglithium cobalt oxide, lithium nickel oxide and lithium manganese oxideas the lithium transition metal oxide which shows the reversibleelectrochemical insertion and extraction reactions of lithium ions intoor from the lithium ion site in the crystal structure, but needless tosay the similar effect can be obtained when using other lithiumtransition metal oxides such as lithium iron oxide, lithium nickelcobalt oxide, lithium nickel vanadium oxide, lithium manganese chromiumoxide and the like. Thus, the present invention is not limited to theabove-described embodiments comprising the positive electrodes made byusing the mentioned compounds as the lithium transition metal oxidewhich shows the reversible electrochemical insertion and extractionreactions of lithium ions into or from the lithium ion site in thecrystal structure.

Further, in the above Examples of the present invention, discussionswere made with reference to the lithium ion conductive amorphous solidelectrolytes of the compositions 0.01Li₃ PO₄ --0.63Li₂ S--0.36SiS₂ and0.05Li₂ O--0.57Li₂ S--0.38SiS₂, but needless to say, the similar effectcan be obtained by using those of other compositions, those containingsulfides such as Li₂ S--GeS₂, Li₂ S--P₂ S₅ and Li₂ S--B₂ S₃, thosecontaining lithium halide such as LiCl--Li₂ S--SiS₂ and LiBr--Li₂ S₂--P₂ S₅, those of the pseudo 4-component systems such as LiI--Li₂S--SiS₂ --P₂ S₅ and LiI--Li₃ PO₄ --Li₂ S--SiS₂, oxide-based ones, andcrystalline lithium ion conductive inorganic solid electrolytes such asLi₃ N, Li₁.3 Sc₀.3 Ti₁.7 (PO₄)₃ and Li₀.2 La₀.6 TiO₃. Thus, the lithiumion conductive inorganic solid electrolyes in the conception of thepresent invention are not limited to those described in the Examples.

Also, the above Examples of the present invention merely describe asolid-state secondary lithium battery assembly construction consistingof two series-connected unit cells housed in a battery container, butobviously the similar effect can be obtained by employing a batterysystem consisting of more than two unit cells connected to each other, asystem in which the unit cells are partly connected in parallel, asystem in which the solid-state secondary lithium batteries housed inthe plural containers are connected in series or parallel, a system inwhich the solid-state secondary lithium batteries are connected to thesecondary lithium batteries using an organic solvent electrolyte, and asystem in which the solid-state secondary lithium batteries areconnected to other secondary batteries such as nickel-cadminumbatteries.

According to the present invention, as described above, a solid-statesecondary lithium battery with excellent charge and discharge cyclecharacteristics can be obtained by incorporating a structure in whichthe amount of lithium ion inserted to the negative electrode until itspotential showing discontinuous change of potential consequent to thelithium ion insertion/extraction reaction changes discontinuously isequal to or smaller than the maximum amount of extraction of lithiumions within the range where insertion/extraction of lithium ions into orfrom the lithium transition metal oxide is reversible.

Also, according to the present invention, a battery assembly withexcellent charge and discharge cycle characteristics can be obtained byusing said solid-state secondary lithium battery at least as one unitcell having the smallest capacity.

Further, according to the present invention, it is possible to betterthe charge and discharge cycle characteristics of the solid-statesecondary lithium batteries by charging the batteries, which showdiscontinuous change of voltage from V1 to V2 with relation to thecharged quantity of electricity, at a voltage below V2.

What is claimed is:
 1. A battery assembly comprising plural electricallyconnected unit cells, which battery assembly has a structure of pluralunit cells connected in series at a part of the battery assembly and aunit cell having at least the smallest capacity among the plural unitcells connected in series is a solid-state secondary lithium batterycomprising (a) an electrolyte layer comprising mainly a lithium ionconductive solid electrolyte, (b) a positive electrode comprising alithium transition metal oxide capable of inducing reversibleelectrochemical insertion and extraction reactions of lithium ions intoor from lithium ion sites in a crystal structure, and (c) a negativeelectrode comprising a material showing discontinuous change ofpotential consequent to insertion and extraction reactions of lithiumions into or from the lithium ion sites in the crystal structure,whereinan amount of the lithium ion inserted, until a potential of the materialshowing discontinuous change of potential consequent to the insertionand extraction reactions of lithium ions discontinuously changes, isequal to or less than a maximum amount of an extracted amount of thelithium ions which is extracted by the reversible insertion andextraction reactions of the lithium transition metal oxide.
 2. Thebattery assembly according to claim 1, wherein the plural unit cells arehoused in a single airtight battery container.
 3. A battery assemblycomprising plural electrically connected unit cells, which batteryassembly has a structure of plural unit cells connected in series at apart of the battery assembly and a unit cell having at least thesmallest capacity among the plural unit cells connected in series is asolid-state secondary lithium battery comprising (a) an electrolytelayer comprising mainly a lithium ion conductive solid electrolyte, (b)a positive electrode comprising a lithium transition metal oxide capableof inducing reversible electrochemical insertion and extractionreactions of lithium ions into or from lithium ion sites in a crystalstructure, and (c) a negative electrode comprising a material showingdiscontinuous change of potential consequent to insertion and extractionreactions of lithium ions into or from the lithium ion sites in thecrystal structure,wherein an amount of the lithium ion inserted, until apotential of the material showing discontinuous change of potentialconsequent to the insertion and extraction reactions of lithium ionsdiscontinuously changes, is equal to or less than a maximum amount of anextracted amount of the lithium ions which is extracted by thereversible insertion and extraction reactions of the lithium transitionmetal oxide, and wherein the material showing discontinuous change ofpotential consequent to the insertion and extraction reactions tolithium ions into or from the lithium ion sites in the crystal structureis at least one metal selected from the group consisting of indium,aluminum, lead, bismuth, antimony gallium, tin, silver, silicon, zinc,cadmium, arsenic and titanium, an alloy of the metals selected from theabove group, or an alloy of lithium and at least one metal selected fromthe above group.
 4. A battery assembly comprising plural electricallyconnected unit cells, which battery assembly has a structure of pluralunit cells connected in series at a part of the battery assembly and aunit cell having at least the smallest capacity among the plural unitcells connected in series is a solid-state secondary lithium batterycomprising (a) an electrolyte layer comprising mainly a lithium ionconductive solid electrolyte, (b) a positive electrode comprising alithium transition metal oxide capable of inducing reversibleelectrochemical insertion and extraction reactions of lithium ions intoor from lithium ion sites in a crystal structure, and (c) a negativeelectrode comprising a material showing discontinuous change ofpotential consequent to insertion and extraction reactions of lithiumions into or from the lithium ion sites in the crystal structure,whereinan amount of the lithium ion inserted, until a potential of the materialshowing discontinuous change of potential consequent to the insertionand extraction reactions of lithium ions discontinuously changes, isequal to or less than a maximum amount of an extracted amount of thelithium ions which is extracted by the reversible insertion andextraction reactions of the lithium transition metal oxide, and whereinthe material showing discontinuous change of potential consequent to theinsertion and extraction reactions of lithium ions into or from thelithium ion sites in the crystal structure is indium, and the amount ofthe lithium ion inserted, until potential of the material showingdiscontinuous change of potential consequent to the insertion andextraction reactions of lithium ions discontinuously changes, is theamount corresponding to the following reaction formula (1):

    In+Li.sup.+ +e.sup.- →In-Li                         (1).


5. A battery assembly comprising plural electrically connected unitcells, which battery assembly has a structure of plural unit cellsconnected in series at a part of the battery assembly and a unit cellhaving at least the smallest capacity among the plural unit cellsconnected in series is a solid-state secondary lithium batterycomprising (a) an electrolyte layer comprising mainly a lithium ionconductive solid electrolyte, (b) a positive electrode comprising alithium transition metal oxide capable of inducing reversibleelectrochemical insertion and extraction reactions of lithium ions intoor from lithium ion sites in a crystal structure, and (c) a negativeelectrode comprising a material showing discontinuous change ofpotential consequent to insertion and extraction reactions of lithiumions into or from the lithium ion sites in the crystal structure,whereinan amount of the lithium ion inserted, until a potential of the materialshowing discontinuous change of potential consequent to the insertionand extraction reactions of lithium ions discontinuously changes, isequal to or less than a maximum amount of an extracted amount of thelithium ions which is extracted by the reversible insertion andextraction reactions of the lithium transition metal oxide, and whereinthe material showing discontinuous change of potential consequent to theinsertion and extraction reactions of lithium ions into or from thelithium ion sites in the crystal structure is selected from the groupconsisting of transition metal oxides, lithium transition metal oxides,transition metal sulfides and lithium transition metal sulfides.
 6. Abattery assembly comprising plural electrically connected unit cells,which battery assembly has a structure of plural unit cells connected inseries at a part of the battery assembly and a unit cell having at leastthe smallest capacity among the plural unit cells connected in series isa solid-state secondary lithium battery comprising (a) an electrolytelayer comprising mainly a lithium ion conductive solid electrolyte, (b)a positive electrode comprising a lithium transition metal oxide capableof inducing reversible electrochemical insertion and extractionreactions of lithium ions into or from lithium ion sites in a crystalstructure, and (c) a negative electrode comprising a material showingdiscontinuous change of potential consequent to insertion and extractionreactions of lithium ions into or from the lithium ion sites in thecrystal structure,wherein an amount of the lithium ion inserted, until apotential of the material showing discontinuous change of potentialconsequent to the insertion and extraction reactions of lithium ionsdiscontinuously changes, is equal to or less than a maximum amount of anextracted amount of the lithium ions which is extracted by thereversible insertion and extraction reactions of the lithium transitionmetal oxide, and wherein the transition metal element of the lithiumtransition metal oxides is at least one element from the groupconsisting of cobalt, manganese and iron.
 7. A battery assemblycomprising plural electrically connected unit cells, which batteryassembly has a structure of plural unit cells connected in series at apart of the battery assembly and a unit cell having at least thesmallest capacity among the plural unit cells connected in series is asolid-state secondary lithium battery comprising (a) an electrolytelayer comprising mainly a lithium ion conductive solid electrolyte, (b)a positive electrode comprising a lithium transition metal oxide capableof inducing reversible electrochemical insertion and extractionreactions of lithium ions into or from lithium ion sites in a crystalstructure, and (c) a negative electrode comprising a material showingdiscontinuous change of potential consequent to insertion and extractionreactions of lithium ions into or from the lithium ion sites in thecrystal structure,wherein an amount of the lithium ion inserted, until apotential of the material showing discontinuous change of potentialconsequent to the insertion and extraction reactions of lithium ionsdiscontinuously changes, is equal to or less than a maximum amount of anextracted amount of the lithium ions which is extracted by thereversible insertion and extraction reactions of the lithium transitionmetal oxide, and wherein the lithium ion conductive solid electrolyte isan inorganic compound.
 8. A battery assembly comprising pluralelectrically connected unit cells, which battery assembly has astructure of plural unit cells connected in series at a part of thebattery assembly and a unit cell having at least the smallest capacityamong the plural unit cells connected in series is a solid-statesecondary lithium battery comprising (a) an electrolyte layer comprisingmainly a lithium ion conductive solid electrolyte, (b) a positiveelectrode comprising a lithium transition metal oxide capable ofinducing reversible electrochemical insertion and extraction reactionsof lithium ions into or from lithium ion sites in a crystal structure,and (c) a negative electrode comprising a material showing discontinuouschange of potential consequent to insertion and extraction reactions oflithium ions into or from the lithium ion sites in the crystalstructure,wherein an amount of the lithium ion inserted, until apotential of the material showing discontinuous change of potentialconsequent to the insertion and extraction reactions of lithium ionsdiscontinuously changes, is equal to or less than a maximum amount of anextracted amount of the lithium ions which is extracted by thereversible insertion and extraction reactions of the lithium transitionmetal oxide, wherein the lithium ion conductive solid electrolyte is aninorganic compound, and wherein the inorganic compound is an amorphouscompound mainly composed of a sulfide.
 9. A battery assembly comprisingplural electrically connected unit cells, which battery assembly has astructure of plural unit cells connected in series at a part of thebattery assembly and a unit cell having at least the smallest capacityamong the plural unit cells connected in series is a solid-statesecondary lithium battery comprising (a) an electrolyte layer comprisingmainly a lithium ion conductive solid electrolyte, (b) a positiveelectrode comprising a lithium transition metal oxide capable ofinducing reversible electrochemical insertion and extraction reactionsof lithium ions into or from lithium ion sites in a crystal structure,and (c) a negative electrode comprising a material showing discontinuouschange of potential consequent to insertion and extraction reactions oflithium ions into or from the lithium ion sites in the crystalstructure,wherein an amount of the lithium ion inserted until apotential of the material showing discontinuous change of potentialconsequent to the insertion and extraction reactions of lithium ionsdiscontinuously changes, is equal to or less than a maximum amount of anextracted amount of the lithium ions which is extracted by thereversible insertion and extraction reactions of the lithium transitionmetal oxide, wherein the lithium ion conductive solid electrolyte is aninorganic compound, and wherein the inorganic compound is synthesizedfrom a material mainly composed of lithium sulfide and silicon sulfide.10. The battery assembly according to claim 3, wherein the plural unitcells are housed in a single airtight battery container.
 11. The batteryassembly according to claim 4, wherein the plural unit cells are housedin a single airtight battery container.
 12. The battery assemblyaccording to claim 5, wherein the plural unit cells are housed in asingle airtight battery container.
 13. The battery assembly according toclaim 6, wherein the plural unit cells are housed in a single airtightbattery container.
 14. The battery assembly according to claim 7,wherein the plural unit cells are housed in a single airtight batterycontainer.
 15. The battery assembly according to claim 8, wherein theplural unit cells are housed in a single airtight battery container. 16.The battery assembly according to claim 9, wherein the plural unit cellsare housed in a single airtight battery container.